Color inkjet printers and other printers employing an inkjet system are well known in the art as a type of recording device. A typical inkjet printer has a recording head in which is formed a plurality of ejection holes arrayed in a sub-scanning direction. A main scan is performed by ejecting ink sequentially from the ejection holes onto a recording medium while moving the recording head in a main scanning direction. Through this main scan, the printer forms a plurality of rasters on the recording medium, where one raster is one line configured of a plurality of dots aligned in the main scanning direction.
After performing this main scan, the inkjet printer next executes a sub scan according to a method such as conveying the recording medium in a direction intersecting (generally orthogonal to) the main scanning direction. After completing the sub scan, the printer again performs the main scan. An image is recorded (printed) on the recording medium by repeatedly and alternately performing the main scan and sub scan.
The recording head in this inkjet printer has a piezoelectric actuator for each ejection hole. During a recording operation, drive voltages are applied to the piezoelectric actuators, deforming the piezoelectric actuators and causing a prescribed amount of ink to be ejected from the corresponding ejection holes.
With increasing demand in recent years to improve the resolution of images printed on printing materials and to increase the printing speed, efforts have been made to increase the density of the ejection holes formed in the recording head and to shorten the time interval between ink ejections.
However, if ink droplets are ejected at shorter intervals from densely arranged ejection holes, the load on each ejection hole becomes severe when the ink droplets must be ejected from all ejection holes in a single main scan. In such a case, the system may not be able to respond completely by the next main scan, making ink ejection in the next scan unstable. Specifically, ink ejection in the next main scan may be irregular due to a less than normal amount of ink being ejected or the shape of the ejected ink droplets being distorted, for example, resulting in nonuniform or improperly formed dots and, consequently, a deterioration in image quality in the printed results.
Japanese unexamined patent application publication No. 2000-218777 describes an inkjet recording apparatus having a thinning multi-pass printing mode. When printing in this mode, image data to be printed in each scan is pseudo-randomized with respect to the number of scans so as to shuffle the order in which the image data is printed. In this way, the printing apparatus reduces image noise while improving image quality.